1. Field of the Invention
The present invention relates to a microwave tube such as a klystron or traveling-wave tube that is used in the amplification and oscillation of a high-frequency signal, and to a microwave tube system that is provided with a power supply device for supplying a prescribed power supply voltage to each electrode of the microwave tube.
2. Description of the Related Art
A microwave tube such as a klystron or a traveling-wave tube is an electron tube that realizes the amplification and oscillation of a high-frequency signal through the interaction between a high-frequency circuit and an electron beam that is emitted from an electron gun. As shown in FIG. 1, such a microwave tube is a construction that includes: electron gun 10 for emitting electron beam 50; helix 20, which is a high-frequency circuit for causing interaction between electron beam 50 that is emitted from electron gun 10 and a high-frequency signal (microwave or millimeter wave); collector electrode 30 for capturing electron beam 50 that is supplied from helix 20; and anode electrode 40 for guiding electron beam 50 that is emitted from electron gun 10 through helix 20.
Electron gun 10 is equipped with: cathode electrode 11 for emitting thermions; heater 12 for supplying thermal energy for causing cathode electrode 11 to emit thermions; and Wehnelt electrode 13 for focusing thermions to form electron beam 50.
A prescribed power supply voltage from power supply device 60 is supplied to collector electrode 30 and electron gun 10 of microwave tube 1 that is shown in FIG. 1, and anode electrode 40 and helix 20 are each connected to the case of microwave tube 1 and thus grounded.
A common negative high voltage (cathode voltage) is supplied from power supply device 60 to Wehnelt electrode 13 and cathode electrode 11 of electron gun 10, and a prescribed voltage that takes the cathode voltage as a reference is supplied to heater 12. In addition, a positive high voltage that takes the cathode voltage as a reference is supplied to collector electrode 30.
Microwave tube 1 also includes a configuration in which the connection between anode electrode 40 and helix 20 is cut and different power supply voltages are supplied to anode electrode 40 and helix 20.
In this type of configuration, electron beam 50 that is emitted from electron gun 10 is accelerated by anode electrode 40 and introduced into helix 20, and then travels inside helix 20 while interacting with the high-frequency signal that is applied as input to helix 20. Output electron beam 50 that is supplied from helix 20 is captured by collector electrode 30. At this time, a high-frequency signal that has been amplified by interaction with electron beam 50 is supplied as output from helix 20.
However, cathode electrode 11 that is provided in electron gun 10 that is shown in FIG. 1 is typically formed of a porous tungsten substrate shaped as a disk that has been impregnated with an oxide of, for example, barium (Ba), calcium (Ca), or aluminum (Al). The oxide (impregnated material) that has been impregnated in this cathode electrode 11 is vaporized by the heat of heater 12 and adheres to Wehnelt electrode 13 and anode electrode 40.
In microwave tube 1, a high voltage of at least several KV is applied between anode electrode 40 and cathode electrode 11 during operation, and when minute protuberances are formed from the impregnated material that adheres to Wehnelt electrode 13 and anode electrode 40, an electric field concentrates at these minute protuberances and an electrical discharge is generated between Wehnelt electrode 13 and anode electrode 40.
When discharge is produced between Wehnelt electrode 13 and anode electrode 40, the form of the electric field that is formed at Wehnelt electrode 13 by the discharge current is disrupted, the path of electron beam 50 that is emitted from electron gun 10 is disturbed, and a portion of this electron beam 50 collides with anode electrode 40 or helix 20. As a result, pulse-shaped noise is generated in the high-frequency signal that is amplified in helix 20.
In addition, the collision of electrons with anode electrode 40 and helix 20 causes the flow of current between cathode electrode 11 and anode electrode 40 and between cathode electrode 11 and helix 20. At this time, the path of electron beam 50 that is disrupted by the discharge does not immediately recover, and the state in which current flows between cathode electrode 11 and anode electrode 40 and between cathode electrode 11 and helix 20 continues for at least several msec. In particular, in the configuration shown in FIG. 1 in which anode electrode 40 and helix 20 are connected, the discharge current that is generated between Wehnelt electrode 13 and anode electrode 40 and the current that is produced by the collision of electron beam 50 with anode electrode 40 and helix 20 all pass through helix 20 and are fed back to power supply device 2, raising the concern that helix 20 will be damaged by excess current (Ihel excess current).
When the form of the electric field that forms at Wehnelt electrode 13 is disrupted by the discharge current, moreover, the diameter of electron beam 50 fluctuates inside helix 20 and irregularities occur in the interaction with the high-frequency signal, leading to variations such as increase in the power consumption and decrease in the amplification performance of microwave tube 1.
As one example of a configuration for dealing with this problem, Japanese Patent Laid-Open Publication No. 301342/1992 (hereinbelow referred to as “Patent Document 1”) proposes a configuration in which an inductance element is serially inserted in the lead line that connects anode electrode and power supply device to suppress discharge that is produced between anode electrode and cathode electrode.
However, the microwave tube that is described in the above-described Patent Document 1 is a configuration in which different power supply voltages are applied to the anode electrode and the helix, and this configuration therefore cannot be easily applied to the configuration shown in FIG. 1 in which the same voltage is applied to anode electrode 40 and helix 20. Even if an inductance element were used, its inductance value is extremely high (Patent Document 1 provides an example of using 24 Henries [H]), and moreover, an inductance element has both large cubic volume and weight and therefore cannot be applied to systems that require compact size and light weight.
The discharge that is produced between Wehnelt electrode 13 and anode electrode 40 of above-described microwave tube 1 can be reduced by the method that is proposed in Patent Document 1 or by modifying the configuration of microwave tube 1, but this discharge is difficult to completely eliminate.
Thus, in a microwave tube system of the prior art, power supply device 60 is provided with Ihel excess current detection circuit 61 for detecting excess current of helix 20 that flows as a result of the discharge between Wehnelt electrode 13 and anode electrode 40 by observing the current that flows between helix 20 and cathode electrode 11 as shown in FIG. 1. Ihel excess current detection circuit 61 supplies alarm signal (Ihel ALARM) output when an excess current that threatens to damage helix 20 flows continuously for a prescribed interval of time or more, and power supply device 60, upon detecting the alarm signal, halts the supply of power to microwave tube 1.
However, microwave tubes 1 are used in, for example, the transmission devices of, for example, satellite communication systems or satellite broadcasting systems in which repair or exchange is difficult, and as a result, in many cases the operation of microwave tube 1 cannot be easily halted despite the output of an alarm signal from the above-described Ihel excess current detection circuit 61. On the other hand, discharge that is produced between Wehnelt electrode 13 and anode electrode 40 may shorten the operating life of microwave tube 1 due to damage to helix 20 caused by excess current or by drops in the insulative capacity between electrodes, and moreover, may bring about system halts due to damage to devices or drops in system performance due to the occurrence of noise. Thus, the protective functions of a microwave tube that employs the above-described alarm signal (Ihel ALARM) cannot be eliminated.
Accordingly, a microwave tube is preferably used while detecting discharge between Wehnelt electrode 13 and anode electrode 40 such that an alarm signal is generated only when discharge is generated frequently or when a large discharge current flows, and not when discharge is produced only infrequently.